The present invention relates to a reducing agent injection valve abnormality detection unit for detecting an abnormality occurring on a reducing agent injection valve that injects a reducing agent in an exhaust pipe to purify nitrogen oxides in exhaust gas, and also relates to a reducing agent supply apparatus.
Conventionally, as one aspect of an exhaust gas purification apparatus for removing nitrogen oxides (hereinafter referred to as “NOx”) in exhaust gas from an internal-combustion engine, an apparatus is practically used which includes: an NOx purification catalyst placed on an exhaust gas passageway and a reducing agent supply apparatus that injects a liquid reducing agent, such as an urea aqueous solution and unburnt fuel, upstream of the NOx purification catalyst.
The reducing agent supply apparatus used in such an exhaust gas purification apparatus includes: a storage tank for storing a liquid reducing agent; a pump for sucking up and pumping a reducing agent stored in the storage tank; and a reducing agent injection valve for injecting the pumped reducing agent in the exhaust pipe. Among them, the reducing agent injection valve has injection nozzles attached facing the inside of the exhaust pipe for directly injecting a reducing agent in the exhaust pipe.
Here, when an urea aqueous solution is used as a reducing agent, an urea aqueous solution is adjusted to a concentration having the lowest freezing point (e.g., 32.5% concentration with the freezing point of about −11° C.) so as not to freeze even in cold climates. However, when the concentration increases due to evaporation of solvent in an urea aqueous solution or the like reason, the freezing point increases to make an urea aqueous solution more likely to freeze. A frozen urea aqueous solution may cause a valve body to be stuck or an injection nozzle to be blocked, leading to a stuck-open or stuck-closed condition in which the injection nozzles are stuck open or stuck closed, respectively. Stuck-closed condition includes a valve body stuck with the valve closed and freezing of an urea aqueous solution in the injection nozzles (it should be noted that hereinafter “stuck-closed condition” includes “valve body stuck with the valve closed” and “blocked injection nozzles” unless otherwise specified).
Regardless of type of a reducing agent, the above-described stuck-open or stuck-closed condition may also be caused by fine particles, such as soot or the like included in exhaust gas, entering the reducing agent injection valve through the injection nozzles and attaching to a sliding part or seating part of the valve body to be solidified thereon.
Injection by the reducing agent injection valve is controlled based on an instructed injection amount determined by calculation depending on the flow rate of NOx exhausted from the internal-combustion engine or the like. So, when the reducing agent injection valve is abnormally stuck, an error may occur between an actual injection amount and the instructed injection amount in spite of control of the reducing agent injection valve depending on the instructed injection amount.
Then, for example, when the reducing agent injection valve is stuck open, too much reducing agent is supplied in the exhaust pipe, which may cause a reducing agent or a derivative component thereof to spill downstream of the catalyst or may cause a reducing agent directly to attach to the inner surface of the exhaust pipe. On the other hand, when the reducing agent injection valve is stuck closed, too little reducing agent is supplied in the exhaust pipe, which may cause the reducing efficiency of NOx to decrease and may make it difficult to collect a reducing agent into the storage tank when the internal-combustion engine stops. As such, various units configured to detect whether or not the reducing agent injection valve is abnormally stuck have been proposed (e.g., see JP A 2008 169770).